In recovering valuable metals from industrial by-products such as flue dusts and various residues that contain not only copper and arsenic but also other metals such as lead, zinc, iron, silver and cadmium, various different methods are used depending upon the lead content and whether the predominant forms of the metals present are oxides (or sulfates) or sulfides. If the metals are in the form of oxides or sulfates, leaching with sulfuric acid is the common method and to be effective, it requires a fairly high lead content. Flue dusts for instance that meet these requirements are leached with sulfuric acid to dissolve out copper, zinc, iron, arsenic and cadmium, with lead being precipitated in the form of a lead sulfate in combination with silver to be used as a feed for lead smelting. The dissolved copper is reacted with hydrogen sulfide to form copper sulfide, or alternatively it is replaced with metallic iron to be separated as precipitated copper for use as a feed for copper smelting. Iron and arsenic remained in the solution are neutralized, oxidized with air and removed as stable arsenic and iron compounds. Zinc is neutralized with ammonia to a hydroxide which is separated and used as a feed for zinc smelting. Cadmium is reacted with hydrogen sulfide to form cadmium sulfide which is used as a feed for cadmium smelting. This method is already in commercial application.
But if this method is applied to by-product materials mainly comprising metals in the form of oxides or sulfates but having a small lead content, metals such as copper and zinc are dispersed rather than concentrated in the leaching residue, and whether the metal to be recovered is lead or copper, a complex procedure is necessary and great loss results. Since most of silver moves together with lead, residue must be treated to recover silver in spite of its low lead content. Even if silver can be recovered by this method, it is still uneconomical because the concentrations of copper and other metals in this residue are low.